1. Field of the Invention
The present invention relates to a toner for electrostatic charge developing used in a copying machine for forming an image using electrophotography, as well as a developer for electrostatic charge developing using the same, and an image forming method.
2. Description of the Related Art
A method of visualizing image information as an image via an electrostatic charge image such as electrophotography is currently utilized in a variety of fields. In electrophotograhpy, this method is one of forming an image via steps of forming an electrostatic charge image on a photoreceptor by electrification and exposure, developing the electrostatic latent image with a developer containing a toner to form a toner image, and transferring and fixing this toner image onto a recording medium.
As a developer used therein, there are two-component developers including a toner and a carrier, and one-component developers using a magnetic toner or a non-magnetic toner alone. A toner is usually prepared by a kneading-pulverizing method in which a thermoplastic resin together with a pigment, a charge controlling agent, and a releasing agent such as a wax is melted, kneaded, and cooled and, thereafter, this is finely-divided, and classified. In order to improve fluidity and cleanability of this toner, if necessary, inorganic particles or organic particles are added to a surface of toner particles in some cases.
On the other hand, in recent years, with the development of a high information society, demand for the provision of information documents produced by various procedures as an image having high quality has been increased, and study of higher image quality has been advanced in a variety of image forming methods. Also in an image forming method using electrophotography, this demand is not exceptional and, particularly in electrophotography, in order to realize a finer image in color image information, achievement of a reduction in toner particle size and a sharp particle size distribution is demanded.
For example, in an image forming device such as a digital full color copying machine or a printer, a color image manuscript is color-separated with each filter of B (blue), R (red) and G (green), and a latent image of a dot diameter of 20 to 70 μm corresponding to the original manuscript is developed by a subtractive color process using each developer of Y (yellow), M (magenta), C (cyan) and Bk (black).
In this method, as compared with a conventional white and black machine, it is necessary to transfer a larger amount of a developer, and it is also necessary for the method to be applicable to a dot having a smaller diameter. For this reason, in a toner utilized in this method, it becomes increasingly important to maintain uniform electrostatic property including dependency of electrification on environment, retention of uniform electrostatic charge, sharpness in a particle size distribution, and a toner intensity. In addition, when a demand for higher speed and energy saving in these image forming devices is taken into consideration, a toner is required to have fixability at an even lower temperature. In view of these circumstances as well, a toner of a small particle diameter having a sharp particle size distribution is demanded.
However, in the conventional kneading-pulverizing method for preparing a toner via pulverizing and classifying operations, when it is attempted to make a diameter of a toner smaller, a particle diameter which can be actually provided from a viewpoint of economy and performance has a limit of about 8 μm in size.
Currently, a method of preparing a toner having a small particle diameter by various methods has been studied, but in a pulverizing-classifying method, a diameter of particles is only reduced while the previous particle size distribution is retained as it is, and it is difficult to improve the particle size distribution properties. For this reason, when a toner having a smaller diameter is prepared by a kneading-pulverizing method, problems including the occurrence of carrier pollution, photoreceptor pollution, and toner scattering due to the presence of toner of a finer powder side component in a particle size distribution become conspicuous, and it is difficult to realize high image quality and high reliability at the same time.
In recent years, as a means which can intentionally control a shape and an inner structure of toner particles, a toner is extensively prepared, particularly, by a wet process. Specifically, a wet sphering method which can control a toner particle shape, a suspension granulating method which can control a surface structure, and a suspension polymerization or emulsion polymerization aggregation method which can control even an inner structure are being studied. In particular, as a means which can intentionally control a toner particle shape and an inner structure, an emulsion polymerization aggregation method is proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 63-282752 and JP-A No. 6-250439, the disclosures of which are incorporated by reference herein).
An emulsion polymerization aggregation method is a method of forming toner particles by mixing a coloring agent dispersion prepared by dispersing a coloring agent in a solvent, in a resin dispersion generally prepared by emulsion polymerization or the like, then, forming aggregated particles corresponding to a toner particle diameter in this mixed solution, and heating this to coalesce the aggregated particles into toner particles.
A more specific process is as follows. First, by generating hetero aggregation in a mixed solution in which a resin dispersion obtained by emulsion polymerization using an ionic surfactant, and a dispersion of a pigment in which the pigment is dispersed with an ionic surfactant having opposite polarity are mixed, aggregated particles of a size of a toner particle diameter are formed. Then, a large amount of an ionic surfactant is added to stop growth of a particle diameter of aggregated particles. Subsequently, toner particles are formed via a coalescing step of heating aggregated particles to a glass transition point of a resin contained in the particles or higher to coalesce the aggregated particles.
In this emulsion polymerization aggregation method, a toner particle shape can be controlled from an indeterminate shape to a spherical shape by selecting a heating temperature condition. The emulsion polymerization aggregation method has the characteristic that a particle size distribution of a toner can be sharpened, and a particle diameter can be reduced and, particularly in the field of color image formation, the method has been studied as a powerful procedure for obtaining a toner, which realizes a highly fine image.
However, upon preparation of a toner, since use of a large amount of a surfactant cannot not be avoided in order to stabilize particles and aggregated particles containing various raw material components in a dispersion, a surfactant remains in a solution containing the toner particles (hereinafter, abbreviated as “toner slurry” in some cases) after completion of a coalescing step.
When a surfactant remains in toner particles, the resulting toner has low electrification and low resistance, leading to occurrence of fog. In particular, adverse effects under high temperature and high humidity are great, and stable developability and transference of the toner cannot be achieved. For this reason, the advantage of the emulsion polymerization aggregation method that a toner having a sharp particle diameter distribution suitable for forming a highly fine image and having a small particle diameter can be obtained, is deteriorated. In addition, pollution of a surface of toner particles with a surfactant reduces fluidity and storability of a toner, leading to reduction in reliability. Therefore, it is required that a surfactant does not remain in toner particles.
Meanwhile, since it is difficult to separate a large amount of a surfactant remaining in a toner slurry, a washing step of washing toner particles obtained via a coalescing step becomes indispensable in a process for preparing a toner utilizing an emulsion polymerization aggregation method.
However, since a conventional washing step has mostly been performed by water washing, it has been impossible to completely remove a surfactant adhered to a surface of toner particles. In addition, when it is attempted to decrease an amount of a surfactant adhered to a surface of toner particles as much as possible, not only does an amount of water necessary for washing become enormous, but also the increase in a washing water amount influences cost.
As a washing method other than such water washing, a method utilizing alkali washing is proposed (see JP-A No. 5-142847, the disclosure of which is incorporated by reference herein). According to this method, it is presumed that solubility of a surfactant adhered to a surface of toner particles in washing water is certainly enhanced by an alkali, and thereby cleanability is enhanced. However, since an excessive alkali component remains on a surface of toner particles, a problem arises in that a toner particle size is destabilized, an electrification level is reduced, and a particle sized distribution is expanded. In order to solve these problems, it becomes necessary to additionally perform water washing after the alkali washing, which is also a problem with regard to a number of steps, time and cost.
Besides an emulsion polymerization aggregation method, a method of utilizing a surfactant as a dispersant is also known in toner preparing methods such as suspension polymerization, suspension granulation, and wet sphering (for example, see JP-A No. 5-313416, the disclosure of which is incorporated by reference herein). However, an amount of a dispersant used in these methods is small, and most of the dispersant is adhered to a toner particle surface. For this reason, surfactant that remains in an interior or on a surface of toner particles after a step of washing toner particles is of such a small amount that problems such as the aforementioned deterioration in electrostatic properties do not arise.
As a resin (binder resin) for a toner, which has been used frequently in recent years, a polyester resin is more preferably used than a vinyl-based polymer, a representative of which is a styrene-acryl copolymer, from a viewpoint of low temperature fixability and fixing strength. In particular, a polyester resin is more preferably used as a resin for a color toner from a viewpoint of transparency as well.
When a toner is prepared utilizing an emulsion polymerization aggregation method, a polyester resin can be used as a resin for the toner. Such a polyester resin can be synthesized from polyvalent carboxylic acid also having a sulfonic acid group, and a polyhydric alcohol such as an alkylene oxide adduct of hydrogenated bisphenol A (for example, see JP-A No. 5-94043, the disclosure of which is incorporated by reference herein).
However, when the aforementioned alkali washing is utilizing in order to wash and remove a surfactant adhered to toner particles, this leads to hydrolysis of a polyester resin itself contained in the toner. For this reason, problems such as reduction in an electrification level of the toner, reduction in strength of the toner, and expansion of a particle size distribution of the toner arise.